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Physical Review Letters

Sub-shot-noise lasers without inversion

Klaus M. Gheri, Daniel F. Walls

Published Jun 8, 1992• 65 citations
Publisher

Abstract

We present an analysis of the quantum fluctuations of a three-level \ensuremath{\Lambda} atomic system which gives rise to lasing without inversion. We find that the intensity fluctuations in the laser output may be reduced 50% below the shot noise limit.
DOI
10.1103/physrevlett.68.3428

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We present a detailed analysis of the noise properties of a potential lasing scheme employing a coherently driven three-level system in the \ensuremath{\Lambda} configuration inside an optical cavity. We derive expressions for the steady-state laser intensity and the width of the intensity fluctuations and show that the output fields exhibit sub-Poissonian statistics for suitable operating conditions. The fundamental mechanism responsible for noise suppression is identified as increased intensity dissipation inside the cavity through emission and absorption by the atoms, which counteracts the intensity fluctuations.
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We demonstrate that the intensity fluctuations of a laser with a Raman gain medium, described by a large number of independent 3-level atoms, can drop well below the shot noise level. In the limit of a good cavity, where one can adiabatically eliminate the atoms, we find an optimum Mandel Q parameter of Q ≈ −1/2 over a large range of parameters, when the laser is operated well above threshold. This leads to a strong suppression of the low-frequency quantum fluctuations in the laser output. In addition we find that due to suppression of spontaneous emission into the lasing mode, the spectral linewidth of the emitted light can be reduced below the Schawlow-Townes limit by more than one order of magnitude.
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